Colon carcinoma is the second most common fatal human malignancy and has served as a prototype for studies of tumor progression. Recently, vascularity has emerged as a limiting factor in solid tumor development and a clinically important feature in the detection, recurrence, and therapy of colon tumors. The Ink4a/Arf locus encodes p16Ink4a (p16) and p19Arf (Arf; alternative reading frame). Both proteins are tumor suppressors and potent mediators of cell cycle arrest and senescence in vitro. The Ink4a/Arf locus is frequently silenced by methylation in colon adenoma and carcinoma. This observation suggests that the locus may suppress colon tumorigenesis, but direct evidence has been lacking. We examined the effect of a deletion of the locus in mice with multiple intestinal neoplasia (Min). We have found that Min colon tumors show accelerated tumor progression in the absence of p16 and Aft. These tumors are larger, show histologic features of carcinoma, and, most prominently, are more vascular. The latter feature is accompanied by increased vascular endothelial growth factor (VEGF) content. We propose here to dissect the molecular basis of the phenotype. In Aims 1 and 2, respectively, we will determine the individual impacts of p16 and Arf on Min colon tumor progression, using mice selectively deficient in each protein. We will analyze expression of p16 and Arf in Min colon tumors and assess whether methylation and silencing of the p16 or Arf promoters correlates with features of tumor progression. We will assess the impacts of p16 and Arf on tumor cell cycle arrest and senescence. In Aim 3 we will explore the significance of the vascular phenotype for tumor progression. We will generate Min mice with targeted deletion of the VEGF gene in the intestinal epithelium, in Ink4a/Arf-null and -wild type backgrounds, and examine the consequences for colon tumor progression. In Aim 4, we will examine the regulation of VEGF expression in Min colon tumor cells in vivo and in vitro and will test whether antibodies directed against VEGF disrupt the ability of tumor cells to recruit vascular endothelial cells. In summary, these studies will define key determinants of colon tumor progression and vascularity, improve our understanding of p16 and Arf function in physiologic settings of tumor suppression, and address the significance of the enhanced vascularity for colon tumor progression.